keep your hands clean, wrap them in plastic wrap when not animating, don't use bright contrasting colors right next to each other. But really there is no way to keep clay puppets perfect, clay animation is a very unique form of animation, because sometimes fixing and scultping between frames takes longer than actually moving a positioning the characters.

If theyre alredy dirty try washing them with water if that doesnt work try getting a butter knife and slicing off a thin layer of the plasticine then flattening out the plasticine underneath if you want to stop the figure from getting dirty the wrap it in gladwrap or put it in a plastic bag this stops dust from getting onto the plasticine.